The present invention relates to forged steel parts (forgings) produced by hot forging followed by nitriding without heat treatment between the forging and nitriding and having a high fatigue limit and an excellent bending toughness, and to a steel composition suitable for use in the production of such forgings.
Forged steel parts for use in automobiles, such as crankshafts, connecting rods, and knuckles, have conventionally been produced from a carbon steel or an alloy steel by successively subjecting a billet or roughly-shaped dummy of the steel to hot forging to give a shape desired for the part, heat treatment for refining (thermal refining) such as quenching-and-tempering, normalizing, or normalizing-and-tempering, then machining, if necessary, into a finished shape, and finally nitriding, as shown in FIG. 1(a). Thus, for forged steel parts required to have good resistance to seizure and galling and improved fatigue limit, thermal refining has been performed prior to final nitriding.
In recent years, however, attempts have been made to eliminate the thermal refining step in view of cost- and labor-saving and problems such as distortion and misalignment caused by thermal refining. Unfortunately, the fatigue limit of a forging of a steel composition produced by forging followed by nitriding without thermal refining (hereinafter referred to as a non-heat treated, nitrided forging) is lower than that of a conventional forging of the same steel composition produced by forging and subsequent thermal refining followed by nitriding (hereinafter referred to as a heat-treated, nitrided forging).
The forging undergoes some thermal distortion during nitriding. Therefore, subsequent to the nitriding step, the distortion caused by nitriding is eliminated by bending correction or straightening, i.e., by statically applying a bending load to the nitrided forging.
The term "bending toughness" of a forging used herein refers to the maximum strain that can be eliminated by such bending correction, i.e., the limiting strain which can be eliminated from the forging without cracking when the forging is subjected to bending correction after nitriding. The greater the limiting strain of the forging, the better the bending toughness thereof. The limiting strain is hereinafter referred to as "bend-correctable strain" for simplicity.
A non-heat treated, nitrided forging, however, is not satisfactory with respect to its bend-correctable strain, i.e., the limiting strain which can be eliminated from the nitrided forging without cracking. Thus, the bend-correctable strain of a non-heat treated, nitrided forging is lower than that of a heat-treated, nitrided forging.
Hot forging is performed by initially heating a steel billet or dummy to a high temperature which is normally above 1100.degree. C., and after being forged at high temperatures, the forged steel is allowed to cool. During such a hot forging process, the austenitic grains in the steel grow to a coarser grain size so that the steel as forged has an internal structure comprising coarse austenitic grains and ferritic phases precipitated in the form of a net along the boundaries of the coarse austenitic grains in a pearlitic matrix. In non-heat treated, nitrided forgings, a forged steel having such an as-forged internal structure is directly subjected to nitriding without heat treatment for refining the internal structure, and this is thought to cause the above-described inferior properties of the non-heat treated, nitrided forgings compared to heat-treated, nitrided forgings.
Under these circumstances, there is a need to develop a steel capable of producing non-heat treated, nitrided forgings having a high fatigue limit and a high bend-correctable strain.
Steels having a high fatigue limit as-forged without thermal refining and subsequent nitriding have been developed as described, for example, in Japanese Patent Application Laid-Open (Kokai) Nos. 4-193931 (1992) and 64-68424 (1989). However, these steels contains a relatively large amount of vanadium (V) which is intentionally added in order to improve the fatigue limit. Since vanadium is a nitride precipitate-forming element, if these steels which have been developed for non-heat treated, non-nitrided forgings are subjected to nitriding with or without heat treatment, the resulting nitrided forgings will suffer from a low bend-correctable strain.